ACTION OF SMALL ARTERIES. 



77 



This is regulated by the medium-sized arteries leading to 

 the parts. In the walls of the small and medium-sized arte- 

 ries are muscle fibers of a different kind from those of the 

 skeleton. These fibers are spindle-shaped, as shown in Fig. 24, 

 and do not have the cross-markings of the fibers of the skele- 

 tal muscles ; they are in consequence called Non-striated, Smooth 



Connective Tissue 



Endothelium 



uclei 



Muscle Fiber 



Fig. 25. Plain Muscle Fiber. Isolated and in wall of Artery. 



or Plain Muscle Fibers. They are arranged circularly in the 

 walls of the arteries. These fibers have, in common with all 

 muscle fibers, the power of shortening, but at a much slower 

 rate than the striated fibers. When they shorten they reduce 

 the size of the artery, and, therefore, for the time, less blood 

 can flow through the artery. When the muscle fibers cease to 

 shorten, the artery widens, and allows more blood to pass 

 through it. 



To illustrate the action of the muscles in the walls of an 

 artery, let the water run through a hose or large rubber tube. 

 Now, if a row of persons take hold of this tube, the grip of 

 their hands is like that of the muscles. When the hands 



